DME is a ground-based navigation system which consists of a network of ground transponders and airborne interrogating units (interrogators). In operation, an interrogator transmits DME pulse pair signals to be received by an intended ground transponder on a predetermined downlink frequency within the DME frequency band of 962 MHz to 1150 MHz. Upon receiving an interrogation pulse pair signal, the ground transponder determines whether the received signal is a valid interrogation signal by checking the spacing between the two pulses in the DME pulse pair signal. If a valid interrogation is detected, the ground transponder transmits a reply signal on a predetermined uplink frequency after a preset delay of approximately 50 μs. The reply signal consists of a pulse pair with a fixed spacing that is transmitted on a different predetermined uplink frequency within the DME frequency band. The specific pairing of interrogation and replying frequencies and the spacing between the pulses in the interrogation and replying pulse pair signals defines the DME channel/mode of the DME operation.
There are 126 frequency pairings (Channel #001˜#126) and four spacing pairings (Mode X W Y Z) allocated for DME operation within the DME frequency band. Each channel consists of an interrogation frequency band and a replying frequency band that are separated from adjacent bands by 1 MHz. The purpose of defining DME channels and modes is to minimize the co-channel interference between adjacent DME transponders. It is important that adjacent DME transponders operate either on a different frequency or use different modes when operating on the same uplink or down link frequency.
Since the DME frequency range includes the uplink and downlink SSR frequency bands, the DME channels that are within these SSR frequency bands need to be reserved from usage for sites whose operating coverage area (including both interrogation and replying) overlaps with the coverage area of an operating SSR. The FAA Next Generation (NextGen) Automatic Dependent Surveillance-Broadcast (ADS-B) surveillance system, which is built upon SSR links, includes DME channels that overlap the SSR frequencies and these overlapping DME channels cannot be assigned to any DME operations.
The interrogation and replying operation between an interrogator (e.g., aircraft) and a ground transponder enables the aircraft to determine a range to the transponder based on the observed round-trip delay between the transmission of the interrogation pulse pair signal and receipt of the replying pulse pair signal. FIG. 1 illustrates the operating principles of legacy DME equipment. The distance from an aircraft (AC) to a DME transponder ground station (DME transponder) is determined by the onboard avionics (DME interrogator) that interrogates the DME transponder ground station. An interrogation signal containing quasi-randomly spaced DME pulse pairs are transmitted by the DME interrogator on a DME downlink frequency to the DME transponder, as shown in FIG. 1. Upon receiving the interrogation signal, the DME transponder determines whether the pulse pair of the interrogation signal is valid and, when the received interrogation signal is valid, the DME transponder replies with a reply signal containing an identical DME pulse pair to the interrogator on a DME uplink frequency after a fixed transponder delay. The DME interrogator receives the reply signal and correlates the received pulse pair in the reply signal with the known pulse pair transmitted in the interrogation signal to determine the total delay time. By subtracting the known transponder delay time (td) from the total delay time, dividing the resulting time delay by two, and then multiplying the result by the speed of light, the DME interrogator determines the range from the DME interrogator to the DME transponder.
A DME interrogator distinguishes DME transponder replies to its own interrogations from replies to other interrogations using the quasi-random spacing of successive pairs. The DME interrogator performs a correlation between the transmitted DME pulse pair interrogation spacing and the received pulse pair spacing from the transponder to determine if the correct quasi-random spacing can be identified in the received reply signal. An example of the quasi-randomly spaced sequence of pulse pairs for a DME interrogation signal and the DME reply signal are shown in FIG. 1. The randomness of the interrogation pulse pair sequence varies from DME transponder to DME transponder. For simplicity, DME transponders often use a random pick of a set of preselected spacing between two pairs of pulses to “stagger” the interrogation pulse pairs rather than arranging the pulse pair positions using truly random positions.
While the main purpose of the DME transponder is to reply to the interrogation signals from aircraft, the DME transponder also broadcasts its identity periodically. In accordance with international standards, approximately every 40 seconds, each transponder broadcasts its station ID using International Morse code in a time period not exceeding 10 seconds. To transmit the station ID, the DME transponder transmits a Morse code dot as a 0.1 to 0.16 second period consisting of pulse pair signals with a fixed rate of 1350 pp/s and a Morse code dash has a period that is three times longer than the Morse code dot.
When there are either no interrogations or very few interrogations, a DME transponder maintains a minimum pulse pair transmission rate of 700 pp/s by randomly transmitting pulse pairs that are not replies to an interrogation. When there are too many interrogations the transponder omits some of the replies and maintains a maximum transmission rate of between 2610 and 2790 pp/s.
After receiving a DME interrogation signal containing quasi-randomly spaced DME pulse pairs that the DME transponder determines is valid, the DME transponder will not respond to any new DME interrogation signals for up to 60 μs. During this “transponder dead time”, the DME transponder will not reply to a second DME interrogation signal if the second DME interrogation signal arrives within 60 μs of the arrival time of the first DME interrogation signal that the DME transponder determines is valid. The purpose of this “transponder dead time” is to suppress unwanted DME interrogations caused by echo or multipath signals. The result of this “transponder dead time” is that no two DME reply signals will be transmitted closer than 60 μs on the DME reply signal.
The main purpose of DME operation is to allow aircraft to identify and obtain a range to a DME transponder. The DME interrogation pulses do not carry any information other than the unique randomness that is only meaningful to the DME interrogator.
FIG. 2 illustrates the operation principle of the prior art of DME-DME Area Navigation (RNAV) in which the position of an AC can be determined by onboard avionics that interrogate nearby DME transponders to obtain the ranges to the DME transponders and, based on the known locations of the DME transponders. The position of the AC can be calculated based on range multilateration techniques by solving the intersection of range spheres at the altitude indicated by the altimeter. This is referred to as DME-DME MLAT in this disclosure.
FIG. 3 illustrates the case where insufficient DME transponder coverage is obtained due to low altitude. In this case only one DME transponder is in sight such that the AC cannot determine its own position using DME-DME MLAT.
FIG. 4 illustrates the case where inadequate DME-DME MLAT position accuracy is obtained due to undesirable AC-DME transponders geometry. The uncertainty becomes greater if the intersection angles of the circles deviates from 90 degrees.
The next generation (NextGen) national airspace system (NAS) relies primarily on GNSS-based surveillance and navigation systems (i.e., GPS) to provide aircraft position information to the ground for surveillance and control purposes and to the air for navigation purpose.
A signal receiver that receives multiple signals from synchronous sources can determine its own location through means of multilateration (MLAT) using signals transmitted on the Secondary Surveillance Radar (SSR) frequencies of 1030±5 MHz and 1090±5 MHz bands. Generally two types of approaches are involved in the MLAT process. The first type of MLAT approach, generally known as the TOA MLAT or rho-rho navigation technique, assumes the transmission times from all transmitters are known to the receiver: hence the range to the transmitter can be calculated from the signal propagation time based on the time of arrival (TOA) of the signal. Given multiple ranges to different ground transmitters the position of the receiver can be solved as the intersection of the range-derived spheres. The second type of MLAT approach, generally known as the time difference of arrival (TDOA) MLAT technique, assumes the exact times of transmission of the signals are unknown to the receiver, but the transmissions are simultaneous or the relative transmission times are known. In most cases, the signals are transmitted simultaneously, but in other cases known delays are purposely introduced to stagger the time of transmission to avoid synchronous garbling of the signals at the receiver. In either case, the receiver uses the signals, TOAs and any known transmission delays to calculate the range difference of the received signals from pairs of transmitters. The position of the receiver is then calculated as the intersection of the range-difference derived hyperboloids.
The FAA NextGen Automatic Dependent Surveillance—Broadcast (ADS-B) surveillance system is built upon SSR links. However, the density of transmitted signals on the 1030 MHz and 1090 MHz SSR frequency bands is causing a significant amount of signal garbling and loss of data, especially in high traffic areas. Therefore, any system transmitting signals would need to transmit at frequencies other than the SSR frequency bands.
In addition, existing GNSS-based aircraft navigation systems can be disrupted by solar storms that cause severe ionosphere delay variations that degrade both GPS and WAAS and affect the L1 and L5 signals. Current correction broadcasting cannot keep up with the rapid variations during times of solar storms. Under these conditions, WAAS-only aircraft, which include many General Aviation (GA) aircraft, will lose their RNAV capability. Still further, in regional areas interference and jamming will cause weak GPS/WASS signals to be undetectable, thereby causing a loss of RNAV capability.
While the DME/DME or rho-rho MLAT technique, with or without inertial measurement unit (IMU), has been deemed an acceptable means for GPS backup for navigation, this approach suffers from the need of intense interrogations and the lack of DME/DME coverage in the current NAS. In addition, the DME/DME or rho-rho MEAT approach will almost certainly receive resistance from general aviation (GA) users due to the relative high cost of installing an on-board DME/DME or DME/IMU capable unit. Another potential option, eLoran, has not yet gained full political support for deployment and use worldwide.
In addition, there are practical concerns regarding intentional and unintentional interference, regional and temporal unavailability of GPS services, GPS avionic malfunctions and the need for a robust and economic backup solution to the GLASS-based surveillance and navigation systems.
What is needed is a system and method that provides an aircraft position determination and navigation capability in the NAS (National Airspace System) as a backup to or to augment the existing GNSS-based aircraft navigation system.